The goal of this long-term study is to quantify changes in body composition that occur with aging and to determine the significance of these changes to health, morbidity, and mortality. As a spin-off of these analyses normative standards are being developed which are age-specific. A comprehensive summary of all studies reporting longitudinal changes in height has been performed and to these studies has been added the results of analyses of height change with aging in men and women from the Baltimore Longitudinal Study of Aging. From these analyses a clear picture emerges of the nature of the height changes with age; results from several confusing reports whose results are clearly far outside the consensus of these studies can be disregarded. The data show surprisingly, that height continues to increase into the 20s, does not change in the 30s and 40s, then begins to fall at an ever increasing rate into old age with height loss being much more rapid in women than in men. Of importance is the fact that the impact of height loss on computation of the Body Mass Index (a commonly used surrogate for obesity) is quite small and does not explain the finding that the BMI associated with lowest mortality increases progressively with age. We have reported a remarkable set of longitudinal observations on 24-hr urinary creatinine excretion, an index of total body muscle mass, obtained over a period of more than two decades. Simple height and weight measurements to compute the Body Mass Index is flawed as an estimate of obesity mainly because body weight obviously includes not only adipose tissue but also the markedly variable inter-individual mass of muscle. The question of a possible contributory role of muscle to the correlative role of BMI to health variables has not been comprehensively examined. Data from the BLSA have provided unexpected results: (1) Muscle mass has no predictive value for mortality at any age and (2) Muscle mass is unrelated to the common coronary disease risk factors (glucose tolerance, plasma lipids, or blood pressure).